Thermoresponsive poly(N-isopropylacrylamide)/dextran hybrid particles were formed by surface-initiated, aqueous atom transfer radical polymerization via the attachment of a polymerization initiator onto dextran micropheres and polymerization of N-isopropylacrylamide. The resulting hybrid particles were about 250 mum in diameter, and showed thermoresponsiveness the average diameter changed from 273 mum to 214 mum when the temperatures was increased from 25degreesC to 40degreesC (22% decrease).